Ferritic stainless steel foil and method for manufacturing the same (as amended)

ABSTRACT

Provided are a ferritic stainless steel foil excellent in terms of corrugating workability, shape change resistance at a high temperature, and manufacturability and a method for manufacturing the steel foil. A ferritic stainless steel foil having a chemical composition containing, by mass % , C: 0.020% or less, Si: 2.0% or less, Mn: 1.0% or less, S: 0.010% or less, P: 0.050% or less, Cr: 10.0% or more and 25.0% or less, Ni: 0.05% or more and 0.50% or less, Ti: 0.14% or more and 0.25% or less, Al: 0.001% or more and 0.10% or less, V: 0.02% or more and 0.10% or less, N: 0.020% or less, and the balance being Fe and inevitable impurities, and a Vickers hardness of higher than 200 and lower than 350.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT InternationalApplication No. PCT/JP2015/004150, filed Aug. 20, 2015, and claimspriority to Japanese Patent Application No. 2014-175033, filed Aug. 29,2014, the disclosures of each of these applications being incorporatedherein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a ferritic stainless steel foil whichis used for, in particular, a catalyst carrier for an exhaust gaspurifying facility.

BACKGROUND OF THE INVENTION

Nowadays, since automobile exhaust gas regulations are being tightened,there is an increased number of cases where a metal honeycomb made ofstainless steel foil serving as a catalyst carrier for an automotiveexhaust gas purifying facility is equipped in an automobile. A metalhoneycomb is capable of realizing a larger aperture ratio and has higherthermal shock resistance and higher vibration resistance than aprevailing ceramic honeycomb carrier. Therefore, the proportion of caseswhere a metal honeycomb is employed is being increased. In particular,in the case where an exhaust gas purifying facility is equipped in alarge automobile such as a truck, since the size of the carrier islarge, a metal honeycomb, which has a high degree of freedom of a shape,is used in many cases.

Such a metal honeycomb has a honeycomb structure formed by stacking, forexample, flat stainless steel foils (flat foils) and stainless steelfoils formed into a corrugated shape (corrugated foils) in alternatinglayers and by fixing the contact points of the flat foils and thecorrugated foils by using a brazing method or a diffusion joiningmethod, and the metal honeycomb whose surface is coated with a catalyticmaterial is used for an exhaust gas purifying facility.

A metal honeycomb is composed mainly of a high-Al-content ferriticstainless steel foil typified by, for example, a 20mass % Cr-5mass %Al-containing stainless steel foil or an 18mass % Cr-3mass %Al-containing stainless steel foil. By adding 2 mass % to 3 mass % ormore of Al to stainless steel, since an oxide film of Al₂O₃ is formed onthe surface of the stainless steel, there is a significant increase inoxidation resistance. In the case of a gasoline-powered automobile, thetemperature inside its exhaust gas purifying facility is raised due tothe temperature of exhaust gas and a catalytic reaction and may reach ahigh temperature of 1000° C. or higher. Therefore, a high-Al-contentferritic stainless steel foil having an Al content of 3 mass % or more,which has very excellent oxidation resistance at a high temperature, isused for a catalyst carrier. In addition, a foil used for a catalystcarrier is required to have excellent shape change resistance at a hightemperature in order to prevent flaking of the supported catalyst.

On the other hand, in the case of a diesel-powered automobile, thetemperature of the exhaust gas is not raised to such a high temperatureas is the case with the exhaust gas of a gasoline-powered automobile,and the maximum end-point temperature is about 800° C. in most cases. Inthe case of vehicles such as farm machines and construction machinesother than automobiles, the highest temperature of their exhaust gasesis even lower. Therefore, a foil is not required to have such excellentoxidation resistance at a high temperature or such excellent shapechange resistance at a high temperature which the 20mass % Cr-5mass %Al-containing stainless steel foil or the 18mass % Cr-3mass %Al-containing stainless steel foil described above has. On the otherhand, in the case of such high-Al-content ferritic stainless steelfoils, although oxidation resistance is excellent, there is a problem inthat, since a hot-rolled steel sheet in the middle of the manufacturingprocess is poor in terms of toughness, there is a decrease inmanufacturability, which results in an increase in manufacturing costs.Also, these foils are poor in terms of workability. Therefore,fracturing tends to occur in the foils when corrugating work isperformed, and there is a case where it is not possible to form thefoils into a desired shape due to spring back. Consideration is alsogiven to performing annealing before forming is performed in order toincrease corrugating workability. However, in the case of a thin foil,since it is difficult to remove surface scale which is generated whenannealing is performed by performing grinding or pickling, brightannealing is generally performed in a reducing atmosphere. Since brightannealing requires high-level atmosphere control, there is a significantincrease in manufacturing costs due to the addition of a brightannealing process. In order to avoid such an increase in manufacturingcosts, a foil is ideally used for corrugating work without beingsubjected to annealing.

In order to solve the problems described above, stainless steel foilswhose manufacturability is increased by decreasing the Al content asmuch as possible have been proposed.

Patent Literature 1 discloses a metal honeycomb made of a stainlesssteel foil manufactured by limiting the Al content in the range of alevel of impurities to 0.8 mass % and by forming a Cr oxide layerinstead of an Al oxide layer at a high temperature in order to increasediffusion joining capability when a carrier is assembled. In addition,Patent Literature 2 discloses a metal honeycomb made of a stainlesssteel foil manufactured by limiting the Al content in the range of alevel of impurities to 0.8 mass % and by setting the Mo content to be0.3 mass % to 3 mass % in order to increase oxidation resistance,diffusion joining capability, and sulfuric acid corrosion resistance.

However, in the case of stainless steel foils described in PatentLiterature 1 and Patent Literature 2, there is a problem in that, sincea difference in thermal expansion coefficient between a Cr oxide layergenerated on the surface of the foil and the base steel is larger thanin the case of an Al oxide layer, creep deformation occurs at a hightemperature, which results in a change in the shape of the foil andflaking of the oxide layer on the surface of the foil. In the case wheresuch deformation or flaking occurs, since a catalyst supported on thesurface of the foil falls off, it is not possible to satisfy theproperties required for a catalyst carrier.

As described above, in the case where stainless steel having a decreasedAl content is rolled into a foil having a thickness of 200 μm or lessand used for a metal honeycomb, there is a large problem of a change inshape at a high temperature. Such ferritic stainless steel is poor interms of shape change resistance when used at a high temperature.

Therefore, in the past, the present inventors invented a ferriticstainless steel foil manufactured by limiting the Al content to 0.01mass % to 1.0 mass % in order to increase manufacturability and byadding chemical elements such as Cu, Nb, Mo, and W in order to increaseshape change resistance at a high temperature (refer to PatentLiterature 3).

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 7-213918

PTL 2: Japanese Unexamined Patent Application Publication No. 7-275715

PTL 3: Japanese Patent No. 5522330 (International Publication No.WO2013/114833)

SUMMARY OF INVENTION

However, in the case of the ferritic stainless steel foil according toPatent Literature 3, solute-strengthening chemical elements typified by,for example, expensive Cu, Nb, Mo, and W are added in order to increaseshape change resistance at a high temperature. Therefore, there has beena demand for increasing manufacturability by decreasing component costsand for increasing corrugating workability without performing annealingby inhibiting a decrease in workability.

Therefore, an object of the present invention is, by solving theproblems described above, to provide a ferritic stainless steel foilexcellent in terms of corrugating workability, shape change resistanceat a high temperature, and manufacturability and a method formanufacturing the steel foil.

The present inventors diligently conducted investigations in order tosolve the problems described above, and, as a result, found a method forincreasing shape change resistance by adding specified amounts of Cr,Ti, and V instead of adding expensive strengthening chemical elements,and found that, with this method, it is possible to obtain a stainlesssteel foil excellent in terms of corrugating workability. The subjectivematter of the present invention includes the following.

[1] A ferritic stainless steel foil having a chemical compositioncontaining, by mass % , C: 0.020% or less, Si: 2.0% or less, Mn: 1.0% orless, S: 0.010% or less, P: 0.050% or less, Cr: 10.0% or more and 25.0%or less, Ni: 0.05% or more and 0.50% or less, Ti: 0.14% or more and0.25% or less, Al: 0.001% or more and 0.10% or less, V: 0.02% or moreand 0.10% or less, N: 0.020% or less, and the balance being Fe andinevitable impurities, and a Vickers hardness of higher than 200 andlower than 350.

[2] The ferritic stainless steel foil according to item [1] above, thefoil having the chemical composition further containing, by mass % ,one, two, or all of Mo: 0.01% or more and 0.50% or less, Cu: 0.01% ormore and 0.30% or less, and Co: 0.01% or more and 0.20% or less.

[3] The ferritic stainless steel foil according to item [1] or [2]above, the foil having the chemical composition further containing, bymass % , one, two, or more of Nb: 0.01% or more and 0.20% or less, REM:0.01% or more and 0.20% or less, Zr: 0.01% or more and 0.20% or less,Hf: 0.01% or more and 0.20% or less, Ca: 0.0003% or more and 0.0020% orless, and Mg: 0.0005% or more and 0.0030% or less.

[4] The ferritic stainless steel foil according to any one of items [1]to [3] above, the foil being used for a catalyst carrier for an exhaustgas purifying facility used for an exhaust gas whose maximum end-pointtemperature is 800° C. or lower.

[5] A method for manufacturing the ferritic stainless steel foilaccording to any one of the items [1] to [3] above, the method including

a process in which hot rolling is performed on a heated steel slab,

a process in which cold rolling is performed on the hot-rolled steelsheet,

a process in which annealing is performed on the cold-rolled steelsheet, and

a process in which cold rolling is further performed on the annealedsteel sheet with a final rolling reduction of 50% or more and 95% orless.

According to the present invention, it is possible to provide a ferriticstainless steel foil excellent in terms of corrugating workability,shape change resistance at a high temperature, and manufacturability anda method for manufacturing the steel foil.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

First, in order to complete the ferritic stainless steel foil to be usedfor a catalyst carrier for an exhaust gas purifying facility for adiesel-powered automobile according to the present invention, thepresent inventors conducted investigations regarding properties requiredto prevent flaking of a catalyst due to shape change in the case of acatalyst carrier for an exhaust gas purifying facility to be used for anexhaust gas whose highest temperature is 800° C. or lower, and, as aresult, found that it is possible to inhibit flaking of a catalyst fromoccurring in the case where a shape change ratio is 10% or less, orpreferably 5% or less, after the catalyst carrier has been held at atemperature of 500° C. or higher and 800° C. or lower for 100 hours inatmospheric air. Here, a method for determining shape change ratio willbe described in detail in the EXAMPLES.

In order to obtain an inexpensive stainless steel foil having sufficientcorrugating workability and manufacturability while satisfying therequired properties described above, by using a ferritic stainless steelfoil in which the contents of Al, which decreases workability andmanufacturability, and expensive chemical elements such as Cu, Nb, andMo are controlled to be as small as possible, the present inventorsconducted close investigations regarding shape change resistance at ahigh temperature and corrugating workability, and, as a result, obtainedthe following knowledge, which resulted in the completion of the presentinvention.

It was clarified that, in order to increase the shape change resistanceof a ferritic stainless steel foil, the final rolling reduction of acold rolling process performed on steel containing specified amounts of,for example, Cr, Ti, and V should be 50% or more and 95% or less, andVickers hardness (HV) should be controlled to be higher than 200. Thisis thought to be because, since a large amount of work strainaccumulates due to an increased rolling reduction of a cold rollingprocess, recrystallization is promoted by a heat treatment in a metalhoneycomb manufacturing process, which results in an increase in thecrystal grain diameter of a foil.

That is, it is thought that, since the amount of high-temperature creepdeformation and an oxidation rate decrease with an increase in crystalgrain diameter, there is a decrease in the amount of deformation whenthe metal honeycomb is used at a high temperature. On the other hand, inthe case where Vickers hardness (HV) is 200 or lower, the effect ofpromoting an increase in crystal grain diameter is insufficient.Therefore, the Vickers hardness (HV) of the ferritic stainless steelfoil according to the present invention is set to be higher than 200, orpreferably higher than 220.

It is preferable that the crystal grain diameter of a ferritic stainlesssteel foil be increased after the steel foil has been assembled into ametal honeycomb structure. This is because there is an increase in costin the case where an annealing process is added in a foil manufacturingprocess in order to increase a crystal grain diameter. Since a heatingtreatment (joining heat treatment) is usually performed in a vacuum of1.0×10 Pa or less or in a reducing atmosphere at a temperature of 800°C. to 1200° C. for 30 seconds or more for performing brazing ordiffusion joining when a catalyst carrier for a metal honeycomb isassembled, the crystal grain diameter should be increased by performingsuch a heat treatment. That is, by performing corrugating work withcrystal grains being non-recrystallized, the crystal grain diametershould be increased due to recrystallization occurring when a joiningheat treatment is performed. Here, the term “a reducing atmosphere”refers to an atmosphere composed of N₂, H₂, or Ar, or a mixture of thesegases.

On the other hand, in the case where an excessive amount of work strainis accumulated, since it is difficult to perform corrugating work, thereis a case where fracturing occurs when forming is performed. Even iffracturing does not occur, there is a case where it is not possible toperform corrugating work to a degree at which a desired shape isachieved due to an increase in the deformation resistance of the foil.From the results of the investigations, it was clarified that, unlessVickers hardness (HV) is 350 or higher, it is possible to performforming without any problem under severe conditions such that themaximum bending radius is 1 mm or less when corrugating work isperformed. Therefore, the Vickers hardness (HV) of the ferriticstainless steel foil according to an aspect of the present invention isset to be lower than 350, preferably lower than 320, or more preferablylower than 300.

Here, a method for measuring the Vickers hardness (HV) of the ferriticstainless steel foil according to the present invention will bedescribed.

First, after having cut a sample having an appropriate size from a foiland embedding the sample in, for example, a resin so that a crosssection parallel to the rolling direction and at a right angle to thefoil surface is exposed, mirror polishing is performed. Subsequently, bydetermining hardness at 5 points in the central portion in the thicknessdirection of this cross section by using a Vickers hardness meter, theaverage hardness of the 5 points is defined as the Vickers hardness (HV)of the foil. The details of, for example, the determining conditions aredetermined in accordance with JIS Z 2244.

[Ferritic Stainless Steel Foil]

Hereafter, the ferritic stainless steel foil according to embodiments ofthe present invention will be described in detail.

First, the chemical composition of the ferritic stainless steel foilaccording to embodiments of the present invention will be described.Hereinafter, % used when describing the constituent chemical elements ofthe ferritic stainless steel foil according to the present inventionshall always denote mass % .

<C: 0.020% or less>

Since there is a decrease in toughness due to large amounts of carbidesprecipitated when hot rolling is performed in the case where there is anincrease in the C content, and since the corrugating workability of afoil decreases with an increase in the C content, the C content is setto be 0.020% or less, or preferably 0.010% or less. It is morepreferable that the C content be as small as possible.

<Si: 2.0% or less>

Although Si is a chemical element which increases oxidation resistance,in the case where the Si content is more than 2.0%, there is a decreasein toughness, and there is a decrease in workability, which results indifficulty in manufacturing. Therefore, the Si content is set to be 2.0%or less, preferably 1.0% or less, or more preferably 0.2% or less.However, in the case where oxidation resistance is increased to a higherdegree, it is preferable that the Si content be 0.05% or more, or morepreferably 0.1% or more.

<Mn: 1.0% or less>

In the case where the Mn content is more than 1.0%, there is a decreasein oxidation resistance at a high temperature. Therefore, the Mn contentis set to be 1.0% or less, preferably 0.5% or less, or more preferably0.3% or less. However, since Mn is effective for fixing S in steel, itis preferable that the Mn content be 0.05% or more, or more preferably0.1% or more.

<S: 0.010% or less>

In the case where the S content is more than 0.010%, there is a decreasein oxidation resistance at a high temperature. Therefore, the S contentis set to be 0.010% or less, preferably 0.005% or less, or morepreferably 0.003% or less. It is even more preferable that the S contentbe as small as possible.

<P: 0.050% or less>

In the case where the P content is more than 0.050%, there is a decreasein oxidation resistance at a high temperature. Therefore, the P contentis set to be 0.050% or less, or preferably 0.030% or less. It is morepreferable that the P content be as small as possible.

<Cr: 10.0% or more and 25.0% or less>

Since Cr is a chemical element which is essential for achievingsatisfactory oxidation resistance and strength at a high temperature,the Cr content is set to be 10.0% or more. However, in the case wherethe Cr content is more than 25.0%, since there is a decrease inworkability, it is not possible to achieve excellent corrugatingworkability, which is one of the targets of the present invention.Therefore, the Cr content is set to be 10.0% or more and 25.0% or less,or preferably 10.0% or more and 20.0% or less. In consideration of abalance between manufacturing costs and oxidation resistance, it is morepreferable that the Cr content be 16.0% or more and 17.0% or less.

<Ni: 0.05% or more and 0.50% or less>

Ni is effective for increasing brazability when a catalyst carrier isassembled. Such an effect is realized in the case where the Ni contentis 0.05% or more. However, in the case where the content of Ni, which isan austenite-stabilizing chemical element, is more than 0.50%, there isan increase in the thermal expansion coefficient of the foil due to theformation of austenite when the oxidation of Cr starts, which results ina problem of, for example, wrinkling or fracturing of a foil. Therefore,the Ni content is set to be 0.05% or more and 0.50% or less, preferably0.08% or more and 0.30% or less, or more preferably 0.10% or more and0.20% or less.

<Ti: 0.14% or more and 0.25% or less.>

Ti is a chemical element which increases workability and oxidationresistance by fixing C and N in steel, and such an effect is realized inthe case where the Ti content is 0.14% or more. On the other hand, inthe case where the Ti content is more than 0.25%, TiN having a largegrain diameter is precipitated. Since the present invention is based onthe assumption that the thickness of the foil is 200 μm or less, andsince such TiN has a grain diameter of several μm to several tens of μm,such TiN causes perforation by penetrating the foil or a decrease inoxidation resistance by penetrating an oxide film. Therefore, the Ticontent is set to be 0.14% or more and 0.25% or less, or preferably0.15% or more and 0.19% or less.

<Al: 0.001% or more and 0.10% or less>

Al has a deoxidation effect. Such an effect is realized in the casewhere the Al content is 0.001% or more. However, in the case where theAl content is more than 0.10%, there is a decrease in the toughness orpickling capability of a hot-rolled steel sheet. Therefore, the Alcontent is set to be 0.001% or more and 0.10% or less, or preferably0.020% or more and 0.060% or less.

<V: 0.02% or more and 0.10% or less>

V is effective for increasing the toughness of a hot-rolled steel sheetand the oxidation resistance of a foil by combining with C and N insteel. Also, V is effective for preventing perforation when foil rollingis performed by inhibiting the precipitation of TiN having a large graindiameter. In order to realize such effects, the V content is set to be0.02% or more. On the other hand, in the case where the V content ismore than 0.10%, there is a decrease in toughness and oxidationresistance on the contrary. Therefore, the V content is set to be 0.02%or more and 0.10% or less, or preferably 0.02% or more and 0.04% orless.

<N: 0.020% or less>

In the case where the N content is more than 0.020%, there is a decreasein toughness, and there is difficulty in manufacturing due to a decreasein workability. Therefore, the N content is set to be 0.020% or less, orpreferably 0.010% or less.

The base chemical composition of the ferritic stainless steel foilaccording to the present invention is as described above, and theremainder which is different from those above is Fe and inevitableimpurities.

Moreover, the ferritic stainless steel foil according to the presentinvention may contain Mo, Cu, Co, Nb, REM, Zr, Hf, Ca, and Mg asselective chemical elements.

<Mo: 0.01% or more and 0.50% or less>

Mo is effective for increasing the high-temperature strength of aferritic stainless steel foil. Also, Mo increases salt corrosionresistance by stabilizing an oxide film formed on the surface of aferritic stainless steel foil. Such effects are realized in the casewhere the Mo content is 0.01% or more. However, in the case where the Mocontent is more than 0.50%, since there is a decrease in the toughnessof ferritic stainless steel, there may be difficulty in manufacturing afoil. Therefore, in the case where Mo is added, it is preferable thatthe Mo content be 0.01% or more and 0.50% or less, or more preferably0.10% or more and 0.30% or less.

<Cu: 0.01% or more and 0.30% or less>

Cu is a chemical element which is effective for increasing thehigh-temperature strength of a ferritic stainless steel foil. In thecase where Cu is added, since there is an increase in the strength of afoil due to the formation of precipitates having a small grain diameter,high-temperature creep deformation, which is caused by a difference inthermal expansion coefficient between an oxide film formed on thesurface of the foil and the base steel, is inhibited. In addition, as aresult of high-temperature creep deformation being inhibited, there isan increase in the shape stability of a ferritic stainless steel foil ata high-temperature, which results in an increase in the adhesiveness ofan oxide film and the adhesiveness of a catalyst.

In order to realize such effects, it is preferable that the Cu contentbe 0.01% or more. However, in the case where the Cu content is more than0.30%, there is a decrease in the oxidation resistance of a ferriticstainless steel foil, and there may be an increase in cost due todifficulty in working. Therefore, it is preferable that the Cu contentbe 0.01% or more and 0.30% or less. In consideration of decreasing thecost, it is more preferable that the Cu content be 0.05% or more and0.25% or less.

<Co: 0.01% or more and 0.20% or less>

Co is effective for increasing manufacturability by increasing thetoughness of a stainless steel foil. Such an effect is realized in thecase where the Co content is 0.01% or more. On the other hand, in thecase where the Co content is more than 0.20%, there may be a decrease inworkability. Therefore, in the case where Co is added, it is preferablethat the Co content be 0.01% or more and 0.20% or less.

<Nb: 0.01% or more and 0.20% or less>

Nb is effective for increasing the high-temperature strength of a foil.Such an effect is realized in the case where the Nb content is 0.01% ormore. However, in the case where the Nb content is more than 0.20%,since there is a rise in recrystallization temperature, an increase incrystal grain diameter is inhibited when a heat treatment for diffusionjoining is performed, which may result in a decrease in diffusionjoining capability. In addition, since Nb is mixed in an oxide film orforms a compound with Fe, there may be a decrease in shape changeresistance at a high-temperature. Therefore, it is preferable that theNb content be 0.01% or more and 0.20% or less, or more preferably 0.02%or more and 0.05% or less.

<REM: 0.01% or more and 0.20% or less>

The term “REM” here refers to Y and chemical elements respectivelyhaving atomic numbers of 57 through 71 such as La, Nd, and Sm, and thecontent of REM refers to the total contents of these chemical elements.Generally, since REM increases the adhesiveness of an oxide film, REM issignificantly effective for increasing the spalling resistance of thefilm. Such an effect is realized in the case where the REM content is0.01% or more. However, in the case where the REM content is more than0.20%, these chemical elements are concentrated and precipitated atcrystal grain boundaries and then melted when heated at a hightemperature, which may result in the formation of the surface defects ofa hot-rolled steel sheet. Therefore, in the case were REM is added, itis preferable that the REM content be 0.01% or more and 0.20% or less,or more preferably 0.03% or more and 0.10% or less.

<Zr: 0.01% or more and 0.20% or less>

As a result of Zr combining with C and N in steel, there is an increasein the toughness of a hot-rolled steel sheet, and there is an increasein workability, which facilitates the manufacturing of a foil. Also, asa result of being concentrated at grain boundaries in an oxide film,there is an increase in oxidation resistance at a high-temperature andin strength at a high-temperature, in particular, shape changeresistance. Such effects are realized in the case where the Zr contentis 0.01% or more. However, in the case where the Zr content is more than0.20%, since Zr forms intermetallic compounds with, for example, Fe,there may be a decrease in oxidation resistance. Therefore, in the casewhere Zr is added, it is preferable that the Zr content be 0.01% or moreand 0.20% or less, or more preferably 0.01% or more and 0.05% or less.

<Hf: 0.01% or more and 0.20% or less>

Hf is effective for increasing oxidation resistance at a hightemperature by increasing the adhesiveness between an oxide film formedon the surface of a foil and the base steel. In order to realize such aneffect, it is preferable that the Hf content be 0.01% or more. On theother hand, in the case where the Hf content is more than 0.20%, theremay be a decrease in the toughness of a hot-rolled steel sheet in amanufacturing process. Therefore, it is preferable that the Hf contentbe 0.01% or more and 0.20% or less, or more preferably 0.02% or more and0.10% or less.

<Ca: 0.0003% or more and 0.0020% or less>

Ca is a chemical element which is effective for preventing nozzleclogging caused by Ti-based inclusions, which tend to be crystallizedwhen continuous casting is performed. Such an effect is realized in thecase where the Ca content is 0.0003% or more. However, in the case wherethe Ca content is more than 0.0020%, there may be a decrease incorrosion resistance due to the formation of CaS. Therefore, in the casewhere Ca is added, it is preferable that the Ca content be 0.0003% ormore and 0.0020% or less, more preferably 0.0005% or more and 0.0015% orless, or even more preferably 0.0005% or more and 0.0010% or less.

<Mg: 0.0005% or more and 0.0030% or less>

Mg has a function of increasing the adhesiveness between an oxide filmformed on the surface of a ferritic stainless steel foil and the basesteel. In order to realize such an effect, it is preferable that the Mgcontent be 0.0005% or more. On the other hand, in the case where the Mgcontent is more than 0.0030%, there may be a decrease in the toughnessof ferritic stainless steel and the oxidation resistance of a ferriticstainless steel foil. Therefore, it is preferable that the Mg content be0.0005% or more and 0.0030% or less.

<Vickers Hardness: more than 200 and less than 350>

As described above, the Vickers hardness of the ferritic stainless steelfoil according to an aspect of the present invention is set to be morethan 200 and less than 350. In the case where the Vickers hardness of aferritic stainless steel foil is 200 or less, it is not possible tosufficiently realize the effect of promoting an increase in the crystalgrain diameter of the ferritic stainless steel foil. In addition, in thecase where the Vickers hardness of a ferritic stainless steel foil is350 or more, there is a case where it is not possible to performcorrugating work to a degree of achieving a desired shape due to anincrease in the deformation resistance of a foil. Therefore, the Vickershardness of the ferritic stainless steel foil according to an aspect ofthe present invention is set to be more than 200 and less than 350. Itis preferable that this Vickers hardness be more than 220. In addition,it is preferable that this Vickers hardness be less than 320, or morepreferably less than 300. In order to control the Vickers hardness ofthe ferritic stainless steel foil according to the present invention tobe more than 200 and less than 350, the ferritic stainless steelaccording to the present invention should be controlled to have thespecified chemical composition as described above, and the final rollingreduction of a cold rolling process should be controlled to be 50% ormore and 95% or less as described below.

Here, it is preferable that this Vickers hardness be determined in thecentral portion in the thickness direction in the cross section of thefoil. More specifically, it is preferable that, by embedding the samplein, for example, a resin so that a cross section parallel to the rollingdirection (cross section at a right angle to the foil surface) isexposed, by performing mirror polishing on the sample, and bysubsequently determining hardness at 5 points in the central portion inthe thickness direction of this cross section by using a Vickershardness meter, the average hardness of the 5 points be defined as theVickers hardness of the foil. At this time, for example, the details ofthe determining conditions may be determined in accordance with JIS Z2244.

The ferritic stainless steel foil according to the present inventiondescribed above can preferably be used for a catalyst carrier for anexhaust gas purifying facility used for an exhaust gas whose highesttemperature is 800° C. or lower.

[Method for Manufacturing a Ferritic Stainless Steel Foil]

In order to manufacture the ferritic stainless steel foil describedabove, ordinary stainless steel manufacturing equipment may be used. Bypreparing molten steel having the chemical composition described aboveby using, for example, a converter or an electric furnace, by performingsecondary refining on the molten steel by using a VOD method or an AODmethod as needed, and by using an ingot casting-slabbing method or acontinuous casting method, a steel slab is obtained. After havingcharged the cast slab into a heating furnace in order to heat the slab,preferably, to a temperature of 1150° C. to 1250° C., the heated slab issubjected to a hot rolling process. By removing the surface scale of thehot-rolled steel strip obtained as described above by performing shotblasting, pickling, and/or mechanical polishing on the steel strip, andby performing cold rolling plural times with annealing (processannealing) which is performed between those cold rolling, a stainlesssteel foil having a foil thickness of 200 μm or less is obtained. Thefinal rolling reduction of the cold rolling process is set to be 50% ormore and 95% or less, or preferably 60% or more and 90% or less. In thecase where the final rolling reduction is less than 50%, there is a casewhere the Vickers hardness is lower than the range in the presentinvention (e.g., more than Hv200 and less than Hv350), which may resultin poor shape change resistance. Therefore, the final rolling reductionis set to be 50% or more. On the other hand, in the case where the finalrolling reduction is more than 95%, the effect of promotingrecrystallization by accumulating work strain becomes saturated, andthere is an increase in the number of rolling operations. Therefore, thefinal rolling reduction is set to be 95% or less. Here, the term “finalrolling reduction” refers to the amount of decrease in thickness at thefinal time cold rolling is performed divided by the thickness before thefinal time cold rolling is performed. In addition, cold rolling isperformed plural times, and the number of times cold rolling isperformed should be at least 2 in total, including one after hot rollinghas been performed and one after process annealing has been performed.In addition, it is preferable that process annealing be performed underconditions of, for example, an annealing temperature of 800° C. to 1100°C. and a holding time of 5 seconds to 10 minutes.

It is preferable that the thickness of the foil be 200 μm or less. Inaddition, in the case where satisfactory vibration resistance anddurability are particularly required for a catalyst carrier for anexhaust gas purifying facility, it is more preferable that the thicknessof the foil be 100 μm to 200 μm. In particular, in the case where a highcell density and a low back pressure are required, it is more preferablethat the thickness of the foil be 25 μm to 100 μm. In consideration ofthe balance between manufacturing costs and properties, it is morepreferable that the thickness of the foil be 40 μm to 150 μm.

As described above, it is possible to obtain the ferritic stainlesssteel foil according to the present invention, which is excellent interms of oxidation resistance, corrugating workability, shape changeresistance at a high temperature, and manufacturability.

EXAMPLE 1

Hereafter, the present invention will be described on the basis ofexamples. By preparing molten steels having the chemical compositionsgiven in Table 1 by using a vacuum melting method, by heating the steelmaterial to a temperature of 1200° C., and by performing hot rolling ina temperature range of 900° C. to 1200° C., hot-rolled steel sheetshaving a thickness of 3 mm were obtained. Subsequently, by performingannealing including holding these hot-rolled steel sheets in atmosphericair at a temperature of 950° C. to 1050° C. for one minute, byperforming pickling, and by performing cold rolling, cold-rolled steelsheets having a thickness of 1.0 mm were obtained. Subsequently, afterhaving performed annealing including holding the cold-rolled steelsheets in atmospheric air at a temperature of 950° C. to 1050° C. forone minute, surface scale was removed by performing pickling.Subsequently, after having performed cold rolling to a thickness of 0.2mm, process annealing was performed by holding the cold-rolled steelsheets in an N₂ atmosphere at a temperature of 950° C. to 1050° C. forone minute. By further performing cold rolling on the cold-rolled steelsheets which had been subjected to process annealing, foils having awidth of 100 mm and a foil thickness of 50 μm were obtained. In thiscase, since the thickness after process annealing had been performed was0.2 mm, and since the final foil thickness was 50 μm, the final rollingreduction was 75%. The corrugating workability of the foils obtained asdescribed above was evaluated by using the method described below.Cross-sectional hardness after rolling had been performed was determinedby using the method described above. In the case of steel No. 22 where Vwas not added, since perforation occurred when rolling was performed toa foil thickness of 50 μm, the sample was not subjected to theevaluations following this rolling process.

TABLE 1 Steel Chemical Composition (mass %) No. C Si Mn S P Cr Ni Ti AlV N Other Note 1 0.011 1.61 0.13 0.002 0.023 11.1 0.12 0.24 0.022 0.0380.014 — Example 2 0.008 0.60 0.13 0.003 0.022 10.9 0.13 0.21 0.025 0.0250.011 — Example 3 0.008 1.51 0.07 0.001 0.023 11.0 0.12 0.17 0.021 0.0560.010 — Example 4 0.007 0.21 0.12 0.003 0.032 11.3 0.18 0.22 0.045 0.0260.012 REM: 0.06 Example 5 0.010 1.40 0.12 0.002 0.031 13.2 0.15 0.200.036 0.022 0.010 — Example 6 0.013 0.28 0.15 0.002 0.034 13.0 0.13 0.240.026 0.031 0.008 — Example 7 0.009 1.53 0.10 0.001 0.038 12.8 0.17 0.170.021 0.031 0.014 — Example 8 0.006 0.21 0.11 0.002 0.029 13.2 0.10 0.200.024 0.023 0.009 REM: 0.05 Example Zr: 0.05 9 0.005 1.70 0.08 0.0020.038 16.0 0.16 0.24 0.025 0.025 0.007 — Example 10 0.004 0.25 0.110.001 0.031 16.1 0.13 0.18 0.023 0.022 0.008 — Example 11 0.005 0.140.12 0.001 0.030 15.8 0.12 0.23 0.030 0.024 0.011 — Example 12 0.0030.19 0.11 0.001 0.037 16.0 0.19 0.22 0.039 0.026 0.009 REM: 0.05 ExampleHf: 0.04 13 0.013 0.12 0.13 0.001 0.039 21.5 0.20 0.14 0.027 0.028 0.008Cu: 0.27 Example 14 0.011 0.11 0.10 0.002 0.030 24.0 0.18 0.18 0.0240.091 0.013 Cu: 0.21 Example 15 0.007 0.10 0.22 0.002 0.025 19.1 0.120.22 0.033 0.032 0.007 Cu: 0.08 Example 16 0.005 1.48 0.15 0.002 0.02816.1 0.17 0.16 0.088 0.025 0.007 Nb: 0.06 Example 17 0.011 1.31 0.150.002 0.028 16.2 0.14 0.18 0.036 0.029 0.011 Mo: 0.26 Example 18 0.0041.42 0.21 0.001 0.030 16.0 0.11 0.18 0.025 0.027 0.009 Co: 0.12 Example19 0.010 0.09 0.17 0.002 0.033 16.2 0.20 0.17 0.029 0.024 0.012 Ca:0.0016 Example Mg: 0.0011 20 0.008 0.11 0.12 0.002 0.025 25.3 0.13 0.180.036 0.028 0.013 — Comparative Example 21 0.010 0.15 0.13 0.002 0.03515.9 0.15 0.12 0.029 0.032 0.007 — Comparative Example 22 0.010 0.150.13 0.002 0.035 15.9 0.15 0.17 0.031 — 0.007 — Comparative Example

(1) Cross-Sectional Hardness

The cross-sectional hardness of the foil was evaluated by determiningVickers hardness after rolling had been performed. After having cut thefoil into a sample having a size of 10 mm (in the rolling direction)×15mm and embedding the sample in a resin so that a cross section parallelto the rolling direction (at a right angle to the thickness direction ofthe foil) was exposed, mirror polishing was performed. Subsequently, bydetermining hardness at 5 points in the central portion in the thicknessdirection of the cross section by using a Vickers hardness meter with adetermining load of 500 g in accordance with JIS Z 2244, the averagehardness of the 5 points was derived.

(2) Corrugating Workability

The corrugating workability of the foil was evaluated on the basis ofthe amount of spring back when corrugating work was performed on thefoil. Corrugating work was performed by passing the foil through the gapbetween two gear-shaped rolls having a maximum bending radius of 0.5 mmand a wave pitch of 2.0 mm. Although the corrugated foil shall have thesame shape as that of the rolls provided that bending work is ideallyperformed without spring back, practically, the portion which has beensubjected bending work is unbended due to spring back. Therefore, a casewhere the value of [(length after work/length before work)×100] (%),which is the ratio of the length after work has been performed to thelength before work is performed, is small was judged as a case ofexcellent corrugating workability where the effect of spring back wassmall. By performing corrugating work on a foil having a width of 100mm, a length of 300 mm, and a thickness of 50 μm, and by calculating thevalue of [(length after work/length before work)×100] (%), a case wherethe value was 70% or less was judged as ⊙, a case where the value wasmore than 70% and 80% or less was judged as ◯, and a case where thevalue was more than 80% was judged as ×. The case of ⊙ or ◯ was judgedas a case where the object of the present invention was achieved.

(3) Shape Change Resistance at a High Temperature

A method for evaluating shape change resistance at a high temperaturewill be described. Since a metal honeycomb is usually used after ajoining heat treatment such as brazing or diffusion joining has beenperformed, shape change resistance was investigated by using a testpiece which had been subjected to a heat treatment simulating such aheat treatment.

First, three test pieces were prepared for each of various kinds ofsteel, where a test piece was prepared by bending a foil having a widthof 100 mm, a length of 50 mm, and a thickness of 50 μm into a circularcylinder shape and by fixing the edges of the foil to each other byperforming spot welding. Subsequently, a heat treatment corresponding toa heat treatment performed in diffusion joining or brazing was performedat a temperature of 1100° C. for 30 minutes in a vacuum of 1.0×10¹ Pa orless. After having heated the test pieces, which had been prepared asdescribed above, at a temperature of 700° C. for 100 hours in a furnacein an atmospheric air, the average size change ratio, that is, theaverage value of [(cylinder length after heating/cylinder length beforeheating)×100] (%) of the three test pieces was determined. A case wherethe average size change ratio was more than 10% was judged as ×, a casewhere the average size change ratio was more than 5% and 10% or less wasjudged as ◯, and a case where the average size change ratio was 5% orless was judged as ⊙. The case of ⊙ or ◯ was judged as a case where theobject of the present invention was achieved.

The results are given in Table 2.

TABLE 2 Cross-sectional Corrugating Shape Change Hardness Workability ofFoil Resistance Vickers (Length after Size Test Steel HardnessWork/Length before Change Piece No. after Rolling Work) × 100 (%)Evaluation Ratio (%) Evaluation Note A 1 265 64.0 ⊙ 4.8 ⊙ Example B 2263 66.8 ⊙ 5.9 ◯ Example C 3 266 62.9 ⊙ 6.4 ◯ Example D 4 265 61.1 ⊙ 4.5⊙ Example E 5 283 67.4 ⊙ 3.9 ⊙ Example F 6 277 67.1 ⊙ 4.9 ⊙ Example G 7285 65.3 ⊙ 4.1 ⊙ Example H 8 282 67.7 ⊙ 3.2 ⊙ Example I 9 287 63.5 ⊙ 4.8⊙ Example J 10 294 72.6 ◯ 4.1 ⊙ Example K 11 291 72.4 ◯ 4.3 ⊙ Example L12 289 69.7 ⊙ 5.1 ◯ Example M 13 296 72.6 ◯ 5.3 ◯ Example N 14 299 76.0◯ 4.5 ⊙ Example O 15 305 74.0 ◯ 3.5 ⊙ Example P 16 293 76.0 ◯ 3.1 ⊙Example Q 17 290 71.0 ◯ 2.9 ⊙ Example R 18 285 75.0 ◯ 2.4 ⊙ Example S 19295 72.0 ◯ 2.1 ⊙ Example T 20 367 85.0 X 6.4 ◯ Comparative Example U 21275 68.8 ⊙ 13.4 X Comparative Example V 22 No test was performed due tothe perforation of the foil. Comparative Example

In the case of the foils of the examples of the present invention,spring back after corrugating work had been performed was small, whichmeans that these foils were excellent in terms of corrugatingworkability. Moreover, the Vickers hardness after rolling had beenperformed was 200 or more, which means that these foils were excellentin terms of shape change resistance.

On the other hand, in the case of test piece T, which was a comparativeexample having Cr content more than the range according to the presentinvention, the hardness after rolling had been performed was higher thanthe range according to the present invention, which resulted in poorcorrugating workability. In the case of test piece U, which was acomparative example having Ti content less than the range according tothe present invention, Fe oxides were formed due to a decrease inoxidation resistance, which resulted in a decrease in shape changeresistance. In the case of test piece V, which was a comparative examplehaving no V content, since perforation occurred when foil rolling wasperformed due to the precipitation of TiN having a large grain diameter,the subsequent tests were not performed.

From the results described above, it is clarified that the foils of thepresent invention were excellent in terms of corrugating workability andshape change resistance at a high temperature.

EXAMPLE 2

In order to investigate the hardness after rolling had been performedand its influences on corrugating workability and shape changeresistance, foils were manufactured from some of the steels (steel Nos.1, 5, and 10) given in Table 1 with various final rolling reductions.The manufacturing conditions other than the thicknesses when processannealing was performed and the final foil thicknesses and the methodsfor evaluating corrugating workability and shape change resistance werethe same as those used in the EXAMPLE 1. The cold rolling conditions andthe results of the evaluation of corrugating workability and shapechange resistance are given in Table 3.

TABLE 3 Cross- Corrugating Shape Cold Rolling Condition sectionalWorkability of Foil Change Thickness Hardness (Length after Resistancein Final Final Vickers Work/ Length Size Process Foil Rolling Hardnessbefore Change Test Steel Annealing Thickness Reduction after Work) × 100Ratio Piece No. (mm) (μm) (%) Rolling (%) Evaluation (%) Evaluation NoteAA 1 0.5 100 80.0 270 65.5 ⊙ 4.5 ⊙ Example AB 1 0.5 50 90.0 291 70.0 ⊙3.4 ⊙ Example AC 1 0.3 100 66.7 258 64.9 ⊙ 4.8 ⊙ Example AD 1 0.3 5083.3 290 70.1 ◯ 4.5 ⊙ Example AE 5 0.5 100 80.0 285 67.8 ⊙ 4.1 ⊙ ExampleAF 5 0.5 50 90.0 301 72.0 ◯ 2.9 ⊙ Example AG 5 0.3 100 66.7 271 66.7 ⊙3.5 ⊙ Example AH 5 0.3 50 83.3 296 71.7 ◯ 3.2 ⊙ Example Al 10 0.5 10080.0 270 68.0 ⊙ 3.6 ⊙ Example AJ 10 0.5 50 90.0 284 73.4 ◯ 3.1 ⊙ ExampleAK 10 0.3 100 66.7 248 67.1 ⊙ 4.1 ⊙ Example AL 10 0.3 50 83.3 271 70.4 ◯5.2 ◯ Example AM 10 0.2 180 10.0 142 62.1 ⊙ 12.9 X Comparative ExampleAN 10 0.2 150 25.0 166 63.1 ⊙ 11.4 X Comparative Example AO 10 0.2 12040.0 185 63.2 ⊙ 11.0 X Comparative Example

Test pieces AA through AL, which had chemical compositions and thehardness after rolling had been performed within the ranges according tothe present invention, were excellent in terms of shape changeresistance.

On the other hand, test pieces AM and AN, which were comparativeexamples having Vickers hardness lower than the range according to thepresent invention after rolling had been performed while having chemicalcompositions within the range according to the present invention, werepoor in terms of shape change resistance.

From the results of the EXAMPLE 1 and the EXAMPLE 2, it is clarifiedthat the foils within the range according to the present invention wereexcellent in terms of corrugating workability and shape changeresistance. In addition, from the EXAMPLE 1 and EXAMPLE 2, it isclarified that the foils within the range according to the presentinvention are capable of decreasing component costs by decreasing thecontents of solute-strengthening chemical elements such as Cu, Nb, Mo,and W and are excellent in terms of manufacturability as a result ofdecreasing the Al content.

According to the present invention, it is possible to manufacture astainless steel foil, which can preferably be used for a catalystcarrier for an exhaust gas purifying facility used for an exhaust gaswhose highest temperature is relatively low such as about 800° C. orlower by using an ordinary manufacturing line for stainless steel, whichhas a marked effect on the industry.

1. A ferritic stainless steel foil having a chemical composition containing, by mass % , C: 0.020% or less, Si: 2.0% or less, Mn: 1.0% or less, S: 0.010% or less, P: 0.050% or less, Cr: 10.0% or more and 25.0% or less, Ni: 0.05% or more and 0.50% or less, Ti: 0.14% or more and 0.25% or less, Al: 0.001% or more and 0.10% or less, V: 0.02% or more and 0.10% or less, N: 0.020% or less, and the balance being Fe and inevitable impurities, and a Vickers hardness of higher than 200 and lower than
 350. 2. The ferritic stainless steel foil according to claim 1, the foil having the chemical composition further containing, by mass % , one, two, or all of Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 0.30% or less, and Co: 0.01% or more and 0.20% or less.
 3. The ferritic stainless steel foil according to claim 1, the foil having the chemical composition further containing, by mass % , one, two, or more of Nb: 0.01% or more and 0.20% or less, REM: 0.01% or more and 0.20% or less, Zr: 0.01% or more and 0.20% or less, Hf: 0.01% or more and 0.20% or less, Ca: 0.0003% or more and 0.0020% or less, and Mg: 0.0005% or more and 0.0030% or less.
 4. The ferritic stainless steel foil according to claim 1, the foil being used for a catalyst carrier for an exhaust gas purifying facility used for an exhaust gas whose highest temperature is 800° C. or lower.
 5. A method for manufacturing the ferritic stainless steel foil according to claim 1, the method comprising performing hot rolling on a heated steel slab to form a hot-rolled steel sheet, performing cold rolling on the hot-rolled steel sheet, performing annealing on the cold-rolled steel sheet, and performing cold rolling on the annealed steel sheet with a final rolling reduction of 50% or more and 95% or less.
 6. The ferritic stainless steel foil according to claim 2, the foil having the chemical composition further containing, by mass % , one, two, or more of Nb: 0.01% or more and 0.20% or less, REM: 0.01% or more and 0.20% or less, Zr: 0.01% or more and 0.20% or less, Hf: 0.01% or more and 0.20% or less, Ca: 0.0003% or more and 0.0020% or less, and Mg: 0.0005% or more and 0.0030% or less.
 7. The ferritic stainless steel foil according to claim 2, the foil being used for a catalyst carrier for an exhaust gas purifying facility used for an exhaust gas whose highest temperature is 800° C. or lower.
 8. The ferritic stainless steel foil according to claim 3, the foil being used for a catalyst carrier for an exhaust gas purifying facility used for an exhaust gas whose highest temperature is 800° C. or lower.
 9. The ferritic stainless steel foil according to claim 6, the foil being used for a catalyst carrier for an exhaust gas purifying facility used for an exhaust gas whose highest temperature is 800° C. or lower.
 10. A method for manufacturing the ferritic stainless steel foil according to claim 2, the method comprising performing hot rolling on a heated steel slab to form a hot-rolled steel sheet, performing cold rolling on the hot-rolled steel sheet, performing annealing on the cold-rolled steel sheet, and performing cold rolling on the annealed steel sheet with a final rolling reduction of 50% or more and 95% or less.
 11. A method for manufacturing the ferritic stainless steel foil according to claim 3, the method comprising performing hot rolling on a heated steel slab to form a hot-rolled steel sheet, performing cold rolling on the hot-rolled steel sheet, performing annealing on the cold-rolled steel sheet, and performing cold rolling on the annealed steel sheet with a final rolling reduction of 50% or more and 95% or less.
 12. The method for manufacturing the ferritic stainless steel foil according to claim 6, the method comprising performing hot rolling on a heated steel slab to form a hot-rolled steel sheet, performing cold rolling on the hot-rolled steel sheet, performing annealing on the cold-rolled steel sheet, and performing cold rolling on the annealed steel sheet with a final rolling reduction of 50% or more and 95% or less. 